Re: [PATCH 2/2] Remove bdi_congested() and wb_congested() and related functions

["NeilBrown" <neilb@suse.de>]

On Mon, 13 Dec 2021, Dave Chinner wrote:
> On Mon, Dec 13, 2021 at 03:14:27PM +1100, NeilBrown wrote:
> > These functions are no longer useful as the only bdis that report
> > congestion are in ceph, fuse, and nfs.  None of those bdis can be the
> > target of the calls in drbd, ext2, nilfs2, or xfs.
> > 
> > Removing the test on bdi_write_contested() in current_may_throttle()
> > could cause a small change in behaviour, but only when PF_LOCAL_THROTTLE
> > is set.
> > 
> > So replace the calls by 'false' and simplify the code - and remove the
> > functions.
> > 
> > Signed-off-by: NeilBrown <neilb@suse.de>
> ....
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 631c5a61d89b..22f73b3e888e 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -843,9 +843,6 @@ xfs_buf_readahead_map(
> >  {
> >  	struct xfs_buf		*bp;
> >  
> > -	if (bdi_read_congested(target->bt_bdev->bd_disk->bdi))
> > -		return;
> 
> Ok, but this isn't a "throttle writeback" test here - it's trying to
> avoid having speculative readahead blocking on a full request queue
> instead of just skipping the readahead IO. i.e. prevent readahead
> thrashing and/or adding unnecessary read load when we already have a
> full read queue...
> 
> So what is the replacement for that? We want to skip the entire
> buffer lookup/setup/read overhead if we're likely to block on IO
> submission - is there anything we can use to do this these days?

I don't think there is a concept of a "full read queue" any more.
There are things that can block an IO submission though.
There is allocation of the bio from a mempool, and there is
rq_qos_throttle, and there are probably other places where submission
can block.  I don't think you can tell in advance if a submission is
likely to block.

I think the idea is that the top level of the submission stack should
rate-limit based on the estimated throughput of the stack.  I think
write-back does this.  I don't know about read-ahead.

NeilBrown